Spanish guns proofed under CIP rules are also suitable for US ammunition with the caveat (in my opinion) that very heavy loads should be avoided in the lighter weight game guns. This isn't because of chamber pressure but rather recoil that can damage stocks, especially side lock guns with a lot of wood removed from the head of the stock to inlet the locks.
My wife and I have 5 Spanish guns: 28 GA BLNE, 2 20 GA SLE's, 16 GA BLE, and 12 GA SLE.
Our maximum loads are based primarily on the recoil that we are willing to stand, rather than the maximum load the gun might be able to stand. Our target and small bird loads are:
28 GA - 3/4 ounce at 1200 fps
20 GA - 7/8 ounce at 1200/1210 fps
16 GA - 1 ounce at 1165 fps
12 GA - 1 ounce at 1180 or if shooting 50 rounds or less in a round of clays 1 1/8 ounce at 1145
For those times that "more" is needed when shooting early season pheasants in SD or sharptails in MT:
20 GA - 1 ounce at 1220 fps
12 GA - 1 1/8 ounce at 1200 fps
12 GA - 1 1/16 ounce at 1325 fps (Bismuth "just in case" a friend and I hunt a Waterfowl Production Area in MT - we haven't yet, the box is still full)
If I can't break a clay or kill a bird with one of these loads I can't do it with a load that might break the gun or my shoulder - my 74 year old shoulder is more delicate.
Several years ago I read a thread on Shotgun World about a cracked AyA No 2 SLE stock - the original poster had been warned that the 1 1/2 ounce at 1440 fps was too heavy for an AyA No 2 but he ignored the good advice he received. The recoil of that heavy, fast load is 54.5 foot pounds - the heaviest load I listed above is 26 foot pounds - the load that cracked the stock had slightly over twice the recoil of the heaviest load we will shoot. Keep your loads reasonable and a Spanish gun in good condition will be fine with SAAMI loads.
Very well said, sir.
I would add the butt stocks of shotguns made as light game guns frequently have internal wood removed from the butt stock at its rear to help balance the gun overall. This (these) newly made stock void(s) are then plugged with a round wood plug. Here is a (poor) image of the butt of such a stock taken from a video showing the shaping of that butt stock:
https://www.dropbox.com/s/j17x4nfrwi1g99i/stock%20plug2.jpg?dl=0
